This invention relates to a signal amplifier circuit using junction field effect transistors.
Recently, attention has been drawn to the use, as an amplifying element for an audio signal amplifier circuit, of a vertical type junction field transistor, as disclosed in U.S. Pat. No. 3,828,230 issued on Aug. 6, 1974 to Nishizawa et al, which has the same current unsaturated characteristics as those of a triode vacuum tube. The conventional junction field effect transistor, as opposed to such a triode type junction field effect transistor, has the same current saturated characteristics as those of a pentode vacuum tube. When any of the current saturated type and current unsaturated type field effect transistor is used in a circuit, a reverse bias must be applied between the gate electrode and the source electrode of the field effect transistor. That is, in case of a p-channel transistor, the gate bias must be positive with respect to the source bias, and, in case of an n-channel transistor, the gate bias negative. In a prior art circuit, to meet this requirement, either a separate bias power source is provided in the gate circuit, or a self-bias circuit comprised of a resistor and a capacitor connected in parallel with each other is provided in the source circuit.
However, in the former method the separate bias power source must have a polarity opposite to that of the power supply source, resulting in a complicated power source circuit arrangement. In the latter method an output voltage amplitude is restricted due to a voltage drop across the self-bias circuit. This makes it unsuitable for use in an amplifier stage with a relatively great amplitude in particular. The use of a common power supply source, in either method, makes it difficult to provide a direct-coupled signal amplifier circuit in which the output of a preceding stage amplifier is direct-coupled to the input of a succeeding stage amplifier.